Bound the holder's measuring; dump the database platform (hq ADR 0233)

Walks run at most daily and stop after ten minutes or two million files; datasets are read from
their counters and large items from their top level only. The database platform's tables are
dumped with pg_dumpall rather than copied as live files.

modules/distribution/module.json

@@ -43,6 +43,7 @@
         "path": "${dir:registry-data}",
         "class": "rebuildable",
         "backup": "none",
+        "measure": "shallow",
         "why": "the artifact store: every image is built again from its source, and too large to copy every night (ADR 0214 left it out)"
       }
     ]